1. Field of the Invention
This invention relates generally to a photographic camera exposure control system with a multipurpose photocell and, more particularly, to a photographic camera exposure control system wherein a single photocell is utilized for three distinct purposes.
2. Description of the Prior Art
In the photographic art, exposure control systems embodying scanning shutter blade elements which operate to vary exposure aperture areas with time during the exposure interval are well known as indicated by U.S. Pat. No. 3,942,183 entitled "Camera with Pivoting Blades", by G. Whiteside, issued Mar. 2, 1976 in common assignment herewith. Such scanning shutter blade mechanisms generally include a pair of counter reciprocating shutter blade elements each having a primary aperture that traverses the optical axis of the camera during the exposure interval. The primary apertures are shaped so that upon overlying one another during counter movement of the blades, there is defined an effective exposure aperture value which increases to a maximum value in a determinate period of time.
Exposure control is provided by a pair of secondary photocell apertures in respective shutter blade elements which admit scene light to a photoresponsive element in correspondence with the scene light admitted to the focal plane during shutter blade movement through an exposure cycle. The output from the photoresponsive element is directed to an integrator circuit which triggers upon reaching an integration level corresponding to a desired exposure value to terminate the exposure interval by returning the shutter blade elements back to their initial scene light blocking position.
Shutter mechanisms of the aforementioned type are often arranged to operate in synchronism with a source of artificial illumination such as an electronic flash or strobe. The shutter blade mechanism and strobe are generally arranged to operate in either a high ambient scene light intensity mode of operation where no artificial illumination is provided or a low ambient scene light intensity mode of operation where the strobe is fired to provide a source of artificial illumination. Under conditions of low ambient scene light intensity where the strobe is expected to be fired, the scanning shutter blade mechanism may be stopped at an aperture value corresponding to the camera-to-subject distance as determined by focusing the objective lens. Systems of the aforementioned type are generally referred to as "follow focus" systems and the maximum aperture to which the scanning shutter blade elements are allowed to open is controlled by the rangefinding or focusing system of the camera in a manner as is more fully disclosed in U.S. Pat. No. 3,977,014, entitled "Follow Focus Exposure Control System with Improved Uniform Trim Control", by P. Norris, issued Aug. 24, 1976 in common assignment herewith.
Such a system need not actually stop the opening shutter blade movement at a maximum effective exposure aperture as determined by the rangefinding or focusing system of the camera, but may, instead, vary the time at which the strobe light is fired during the opening shutter blade movement as determined in correspondence with the rangefinding or focusing system of the camera. Since the duration of the strobe light is short in comparison to the time required for the shutter blade mechanism to scan from its scene light blocking position to its maximum exposure aperture defining position, the aperture defined by the shutter blade elements at the instant of strobe firing for practical purposes constitutes the effective aperture by which the exposure occurs. Such a system is more fully described in U.S. Pat. No. 3,478,660, entitled "Photographic Apparatus with Flash Exposure Control System", by E. Land, issued Nov. 18, 1969 in common assignment herewith. Range responsive flash firing systems of the aforementioned type can also be utilized in conjunction with sonar rangefinding devices as more fully described in U.S. patent application Ser. No. 898,546, entitled "Range Synchronized Flash Photographic Apparatus and Method for Achieveing Optimum Exposure", filed Apr. 21, 1978 in common assignment herewith.
Exposure and artificial illumination control systems of the aforementioned type are generally intended to provide a source of artificial illumination only during the low ambient scene light intensity mode of operation. However, there may also be conditions during high ambient scene light intensity modes of operation wherein it becomes desirable to provide artificial illumination to fill in the photographic subject against a brightly back lit scene as is more fully disclosed in U.S. Pat. No. 4,023,187, entitled "Exposure Control System with Fill Flash Race Condition", by E. Shenk, issued May 10, 1977 in common assignment herewith. The aforementioned artificial illumination control system operates under conditions of low ambient scene light intensity to fire the strobe light at a predetermined time period subsequent to the initiation of the exposure interval and under conditions of high ambient scene light intensity to fire the strobe light as a consequence of the time integration of the scene light intensity incident to the photoresponsive element reaching a predetermined value.
In such exposure and artificial illumination control systems, it may be preferable to automatically determine whether the ambient scene light intensity is high or low prior to exposure and to thereafter pursue an exposure control mode of operation particularly suited to the ambient scene light condition so detected. Toward this end it would ordinarily be expected to provide a separate preexposure brightness measuring circuit including a photoresponsive element. In order to thereafter directly control the photographic exposure interval, it is necessary to also detect the scene light admitted to the exposure plane either directly or indirectly through corresponding photocell apertures which might require an additional photoresponsive element. In addition, in order to fire a strobe light at a predetermined period subsequent to the initiation of the exposure interval, it becomes necessary to provide some means for signalling the exposure control circuit at the commencement of the exposure interval. Toward this end it is convenient to utilize an LED, photocell combination as more fully disclosed in U.S. Pat. No. 3,628,437 entitled "Photographic Camera" issued Dec. 21, 1971.
Therefore, it is a primary object of this invention to provide an exposure and artificial illumination control system wherein: the ambient scene light intensity is initially measured prior to the photographic exposure interval, a signal is thereafter provided to indicate the commencement of the exposure interval by detecting the light from an LED, and the duration of the exposure interval is thereafter controlled by detecting scene light in correspondence with the scene light admitted to the film plane, all of which is accomplished through the use of a single photoresponsive element or cell.
Other objects of the invention will be in part obvious and will in part appear hereinafter. The invention accordingly comprises a mechanism and system possessing the construction, combination of elements and arrangement of parts which are exemplified in the following detailed disclosure.